Multiple wireless communication standards have been formulated or considered in recent millimeter wave wireless communication using a 60 GHz band. Major wireless LAN/wireless PAN standards that do not require licenses, for example, include WiGig, IEEE 802.15.3C, Wireless HD and ECMA-387. Formulation of IEEE 802.11ad standard is also in progress.
Multiple wireless schemes (for example, a single-carrier scheme and an OFDM (Orthogonal Frequency Division Multiplexing) scheme) also coexist for intended applications in each standard.
Multiple systems corresponding to the respective multiple wireless communication standards coexist, and the multiple wireless schemes also coexist in each system. Once the millimeter wave wireless communication is popularized, it is possible that multiple different wireless schemes may often be used in proximity to one another. The multiple systems can simultaneously communicate in the same space by each using a different frequency channel.
The frequency channels available in the 60 GHz band, however, are limited to three or four channels. Once the millimeter wave wireless communication is popularized, it is expected in not a small number of cases that multiple different systems use the same frequency channel. A concern is that interference may occur among the systems and communication performance may be degraded in each system.
In order to avoid the interference, firstly, interference signals from heterogeneous systems to a target system need to be detected. Carrier sense with power (hereinafter simply represented as “carrier sense”) has been widely used as a conventional signal detection method. The carrier sense is a method of detecting signals by detecting power.
Specifically, in the carrier sense, power of a received signal is measured, and if a value of the measured power exceeds a predetermined threshold, it is recognized that the signal is detected. The carrier sense is characterized by wide applicability regardless of the class of the signal. In contrast, the carrier sense has disadvantages as follows.
In other words, noise is indistinguishable from the signal, depending on the power. If the predetermined threshold is set to be low to enhance detection sensitivity, false detection, in which the noise is incorrectly detected as the signal, is likely to occur. In contrast, if the predetermined threshold is set to be high to prevent the false detection, the detection sensitivity is degraded.
With the carrier sense having the above disadvantages, it may possibly be difficult to satisfy the level of interference detection sensitivity required for receiving signals modulated with multilevel modulation that has been increasingly used in recent years.
In other words, in recent wireless communication, the multilevel modulation is often used due to increase in transmission rates. In the communication using the multilevel modulation, data errors are likely to occur even with a low level of interference. Accurate detection of such a low level of interference is also required for effective avoidance of the interference.
There is a technique using a correlation among signals, as a signal detection method having signal detection sensitivity higher than that of the carrier sense. This technique is broadly divided into a cross-correlation method and an auto-correlation method. The cross-correlation method detects a signal to be detected, based on a correlation value between a preamble part included in a received signal, and a candidate for a known pattern signal used in the preamble part. The auto-correlation method detects the signal to be detected, based on a correlation value between preamble parts of a first signal and a second signal, which are provided by replication of the received signal.
A periodic signal including repetition of a specific signal pattern is often used in the preamble part. In the auto-correlation method, periodicity of the periodic signal is used for the signal detection. The signal detection sensitivity of the auto-correlation method is generally lower than that of cross-correlation detection, while it is higher than that of the carrier sense. This is because, with the signal detection sensitivity of the auto-correlation method, the noise is distinguishable from the signal based on the periodicity of the periodic signal.
Unlike the cross-correlation method, a receiver does not need to know the above specific signal pattern in the auto-correlation method. Accordingly, a receiving apparatus can be implemented in a simple configuration. Moreover, the auto-correlation method needs to detect just waveform periodicity, and therefore does not require processing of the received signal according to the symbol rate of the interference signal. The auto-correlation method has an advantage of easy applicability to the signal detection also for the heterogeneous systems having different symbol rates or modulation schemes.
The signal pattern of the periodic signal used in the preamble part has been defined in each of the multiple wireless communication standards associated with the above described millimeter wave wireless communication. A period of the signal pattern to be used, however, is common to some wireless communication standards. The number of variations of the period of the periodic signal used in the preamble part is relatively smaller than that of the signal pattern. Accordingly, an auto-correlation detector for major periods is provided in the receiving apparatus, which thus can widely detect interference signals from a wide variety of the heterogeneous systems.
FIG. 1 is a diagram provided for describing the auto-correlation method. FIG. 1A illustrates the basic configuration of the auto-correlation detector. FIG. 1B is a diagram illustrating an image of an auto-correlation process.
In the auto-correlation detector illustrated in FIG. 1A, a second signal, in a first signal and the second signal that have been provided by distribution of the received signal, is delayed for a predetermined time by a delay device (delay). The predetermined time corresponds to the period of the periodic signal used in the preamble part of the signal to be detected. The first signal is multiplied by the delayed second signal in a multiplier. The auto-correlation detector of FIG. 1A is provided with a simple multiplier, which, however, may be a complex multiplier. This is because complex baseband signals are generally handled, and multiplication of complex conjugates is executed.
A result of the multiplication obtained in the multiplier is integrated in an integrator for a predetermined period. A correlation value is thereby obtained.
An absolute value of the obtained correlation value is calculated by an absolute value calculation section. In a comparator, the calculated absolute value of the correlation value is compared with a predetermined threshold, and a signal that depends on a result of the comparison is outputted.
Here, the correlation value obtained from the complex baseband signal is a complex number. In an ideal state where the period of the periodic signal used in the preamble part included in the received signal is identical to the delay time given to the second signal in the delay device, however, the resultant correlation value is a positive real number.
In contrast, for example, if phase rotation occurs due to a cause of error of clock deviation, the resultant correlation value may not necessarily be the positive real number. Instead of direct use of the correlation value obtained in the integrator, the absolute value of the correlation value is herein used for the determination. If the cause of error is assured to be sufficiently small, however, a correlation component is substantially identical to a real component, while an imaginary component, for example, is caused by noise. Instead of the use of the absolute value of the correlation value, the real component of the correlation value may be used for the determination.
In other words, the absolute value of the correlation value or the real component of the correlation value is inputted to the comparator, and compared with the predetermined threshold. If the input value is larger than the predetermined threshold, it is determined that the comparator has detected the signal.
In the auto-correlation detector, appropriate setting of the threshold is required to ensure highly-sensitive detection of weak signals with as little false detection as possible. The false detection means incorrect detection of the noise as the signal to be detected, even though the signal to be detected is not received.
With a fixed threshold, the false detection occurs if background noise has fluctuated. The level of the background noise significantly varies and fluctuates, for example, due to variations, temperature characteristics and fluctuations over time in a high-frequency analog circuit, or noise caused by an internal clock circuit. In a receiver using automatic gain control (AGC), the level of the baseband signal and the level of a noise component in the baseband signal significantly fluctuate depending on the level of an input signal.
In particular, in a system in which a packetized signal is transmitted as a transmission frame, fluctuations in the level of the received signal in time, due to the AGC, are extreme in the beginning part of the transmission frame. Accordingly, in order to prevent the false detection in the case of the fixed threshold, the threshold needs to be set to a sufficiently large value. In such setting, the weak signals are not detected as described above, which leads to degradation of the detection sensitivity of the auto-correlation detector.
As a method of maintaining good detection sensitivity while preventing the false detection, there has been proposed a method of setting the value of the threshold based on measured received power, or a method of normalizing the correlation value with the measured received power, and determining the presence or absence of the signal based on the normalized correlation value (for example, Patent Literature (hereinafter, abbreviated as PTL) 1).
FIG. 2 is a diagram provided for describing an auto-correlation detector disclosed in PTL 1. FIG. 2A illustrates the configuration of the auto-correlation detector in PTL 1. FIG. 2B is a diagram illustrating an image of a process in the auto-correlation detector. In a periodic signal of FIG. 2B, a part of a first period is denoted by S1, while a part of a second period is denoted by S2.
As illustrated in FIG. 2B, a correlation between a first signal and a second signal, which have been provided by distribution of a received signal, is obtained. Part S1 and part S2 of the first signal, as well as those of the second signal are targets to be processed in a correlation operation. Since the second signal, however, has been given a delay for one period of the periodic signal, the correlation operation for the first signal and the second signal is actually the correlation operation for part S2 of the first signal and part S1 of the second signal.
Meanwhile, since the targets to be processed in the correlation operation are part S1 and part S2 of the first signal, as well as those of the second signal, a power observation period to be used for the normalization is also a period corresponding to both S1 and S2. The correlation value is normalized with an average value of the power in the power observation period, and the presence or absence of the signal is determined based on the normalized correlation value.